U.S. Pat. No. 4,205,647 describes a fuel fractioning system for creating a "multi-regional" stratified charge in the intake system of an internal combustion engine. The multi-regional stratification is assumed to remain intact with minimum mixing when it is drawn from the intake system into the combustion chamber of the engine. The aim is to create a large number of discrete pockets of charge containing different fuel types and fuel concentrations randomly spaced from one another and distributed throughout the volume of the combustion chamber. Each pocket may be ignitable by a spark but must be ignitable by compression ignition. The pressure waves created by the ignition and combustion of these pockets are individually small and scattered out of phase with one another thereby avoiding the abrupt and intense pressure rise which is characteristic of a conventional spark ignition engine undergoing knocking combustion when a large proportion of its charge auto-ignites all at once.
In the above proposal, the positions of the discrete pockets in relation to a spark plug (if present) are randomised in space and time and will not be repeatable for every engine cycle. If a spark is fired, there is a probability that a pocket near the spark may not be ignitable by spark ignition but this pocket will later be ignitable by compression ignition. If a pocket is ignitable by a spark, the combustion that follows may reach an adjacent pocket and continue by flame propagation if the adjacent pocket is also ignitable. On the other hand, if the adjacent pocket is too lean, the flame propagation will stop but the pocket will later be ignitable by compression ignition. The pressure wave created by the combustion of any one pocket, being small and spread out across the whole combustion chamber, will be very weak and will not be sufficient to cause compression ignition of the adjacent pockets, though these pockets will be ignitable later by the bulk compression of the charge driven by the engine piston.
It is clear in the above proposal that the spark timing cannot be used as a reliable control of the ignition timing of the overall charge. Instead, the effective ignition timing of the charge is controlled by the average compression ignition time delay characteristics of the individual pockets and it is the bulk compression ratio of the engine that ultimately determines the timing of the auto-ignition. The disadvantage however is that the effective ignition timing of the multi-regional stratified charge of the above proposal will vary with temperature, pressure and composition of the charge during the compression process, occurring at any time when the charge is ready to auto-ignite, but not necessarily at a time within the engine cycle appropriate for optimum thermal efficiency. This is to be contrasted with a conventional spark ignition engine or compression ignition (diesel injection) engine where the ignition timing of the charge can be chosen at will over a wide range within the engine cycle to meet the requirement of optimum thermal efficiency by simply imposing at the desired timing a spark or a fuel injection, as the case may be.
The foregoing discussion is based on the initial postulation made in the above proposal that the multi-regional stratification created in the intake system of the engine will remain intact with minimum mixing when it is drawn from the intake system into the combustion chamber. In reality this is almost impossible to achieve because the intake charge will be stretched and squeezed as it flows past the intake valve and will be overlapped and agitated as it is trapped and compressed in the engine cylinder. The multi-regional stratification that may be present in the intake system will be substantially well mixed by turbulence and vortex break-up by the time the charge is trapped and compressed in the combustion chamber. Consequently, the above proposal is an idealised concept which is difficult to reproduce in practice.